It has been found that treatment by heating of p-vinylphenol polymers in the absence or presence of a radical initiator or an accelerator of an acid results in the weight average molecular weight of the polymers being increased without too much changing of their number average molecular weight so as to yield reformed polymers having an expanded molecular weight distribution and suitable branching moieties. These findings have been described in Japanese Patent Application (OPI) No. 58407/85 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application open to public inspection").
The reformed polymers obtained by the above described method have improved processability, flexibility and the like, however, they have drawbacks in that they are deeply colored. Accordingly, it is difficult to use the reformed polymers obtained in fields where their colorings are disliked. For example, reformed polymers having a specific molecular weight and molecular weight distribution are extremely useful as photosensitive resin, particularly as microphotoresist materials. However, such polymers cannot often be used due to their color at visible and ultraviolet regions.